So I started learning a bit about lapidary in order to be able to handle my own screw up when I accidentally gave my players 16,474 carats of raw sapphire (remember, 1 carat = 0.2 grams).

And the more I learned... the more the gems in spell components becomes really, deeply weird.

Let's take a look at Revivify. Revivify requires a diamond worth 300 gp. That's all it says.

Now, generally, a solid, visible diamond such that its not that easy to lose would be about 2-5 carats.

A 5 carat diamond, in a round cut, has a diameter of 11.1mm. Or 1.11 cm. Or 0.00111m. Or the size of your pinkie fingernail.

In terms of treasure, a 5000 gp diamond is all you can find in the DMG for treasure.

It seems reasonable to peg that 5 carat diamond to the 5000 gp diamond, right?

That then means you have a diamond for revivify being 6% the size of the 5 carat diamond. That is a speck of dust.

But, this is where things get weird because of subjectivity. Now, a flawless, or nearly flawless 5 carat diamond would absolutely be worth 5000gp if you can find a buyer.

But what I learned when reading up on lapidary is all the qualities of a gemstone that can affect its perceived value.

First, we have the qualities inherent to the stone - color, inclusions, size. Color might seem weird, but even white diamonds have a color grade, from very faint yellow (M-Z grade) to flawless (D grade). Very flawless diamonds, even in rough state, are very rare, even when you account for the debeers cartel. The majority of jeweler-grade diamonds are graded within the K-L grades. Below those color grades, the diamonds are used for industrial processes - diamond grit saws or diamond polishing compound, for example.

But, many gemstones have color that varies across different axis. An example would be a low-grade tanzanite rough I saw, where in one direction, its got the clear green color, but in the perpendicular direction, it looks clouded and dark. Yellow shading into black. This is known as C-axis color (I think, this is from memory).

So, you have color, and then inclusions, and size. Big gemstones are much, much rarer than smaller gemstones because of the second property - inclusions. These are non-crystalline, or different minerals, included within the body of the pure crystal being considered for a gemstone. Typically lapidarists cut around the inclusions to make roughs for further work.

Then you have the properties imparted by the skill of the lapidarist - cut, window, polish. The cut is where you have the gem cut to maximize the refraction of light through it, for a pleasing, regular shape. The 'window' refers to the inherent refractive properties of that specific stone, and if the lapidary misjudges or miscuts the window, then the gem loses its refractive properties and becomes basically colored glass. Polish is pretty obvious.

So, a gemstone, in its abstract has multiple properties that affect its perceived or subjective value of a gemstone.

So wherefore do magic spells require a specific value of a diamond or other gemstone?

What happens if you get a small diamond, and then unbeknownst to you, the price of diamonds crashes. Does that mean the revivify spell fails for an unknown reason?

What I propose is that the diamond has to be worth 300 gp to the caster. So you could feasibly get a softball sized hunk, that is cloudy and full of inclusions, bare of any work, and value it at 300gp, while another caster might dismiss that as even being a gem worthy of use in the spell.

Now, you probably don't want to have your players carting around .3 carat diamonds. You can double the size if you affect the following qualities from this list: color, inclusions, cut. Make it cloudy, and a 0.6 carat gem, then add an inclusion of a speck of dirt in the middle, and its now 1.2 carats in size. Make it a cabochon, and it could be a 2.4 carat gemstone, which is about ~5mm in size. Or it could be a 10mm rough, with a divot.